The present invention relates to a scanning type optical device of the tandem type, and an image formation apparatus provided with the scanning type optical device of the present invention.
Examples of image formation apparatuses include a digital color image formation apparatus such as a digital color copying machine, in which a writing operation is performed with respect to each of rotary photosensitive members with scanning beams corresponding to images of a plurality of different colors by scanning type optical devices independently disposed in a manner corresponding to the photosensitive members, respectively, and having a plurality of writing unit, so as to form electrostatic latent images, which are then developed with a toner of a corresponding color by a plurality of developing units, respectively, and thereafter, the developed toner images are superimposed on and transferred onto a recording medium (i.e., a transfer member), thus forming a color image.
The writing unit for use in the above-described digital color image formation apparatus emits a writing beam (i.e., an optical flux) from a light source such as a semiconductor laser, the beam is used as a scanning beam via an optical part consisting of a writing lens or the like, and then, the photosensitive member serving as a member to be scanned is scanned with the scanning beam, thus forming an electrostatic latent image in accordance with image information on the photosensitive member.
In the lens serving as the optical part disposed in the above-described writing unit, a beam spot diameter may be varied at each position in a main scanning direction of the beam or optical characteristics such as a light intensity may be varied caused by non-uniformity inside the lens, variations in part precision in forming the shape of a surface or variations in shape influenced by temperature variations.
The above-described variations such as optical characteristics at the positions in the main scanning direction or the like are critical even in a monochromatic image formation apparatus since they cause distortion or non-uniformity of an image. In particular, they have been significantly critical in a color image formation apparatus since they cause a difference in hue (tint) to markedly reduce the quality of a color image.
In view of this, in order to keep the high quality of the color image, the optical characteristics of the scanning beam are measured at a plurality of positions in the main scanning direction, it has been necessary to correct the optical characteristics based on the measurement result in such a manner as to prevent great variations in optical characteristics at the positions in the main scanning direction.
In the conventional image formation apparatus having the scanning type optical device which performs a writing operation by using a beam as disclosed in Japanese Patent Application Laid-open No. H6-320786, a first laser beam detecting sensor is disposed at one end in a scanning region which is out of an image region of a laser beam, and further, a second laser beam detecting sensor is disposed at the other end in the scanning region, and thus, a corrected writing frequency is calculated based on a time after the first laser beam detecting sensor detects the laser beam till the second laser beam detecting sensor detects the laser beam or a count by a clock, so as to correct a writing clock frequency.
However, since the image formation apparatus disclosed in Japanese Patent Application Laid-open No. H6-320786 is configured such that the writing clock frequency is corrected by using the first laser beam detecting sensor disposed at one end in the scanning region and the second laser beam detecting sensor disposed at the other end in the scanning region, there has arisen a problem that the frequency cannot be corrected at an intermediate portion between the first laser beam detecting sensor and the second laser beam detecting sensor although a difference in height between a real image and an ideal image around both of the ends in the scanning region can be made zero.
Therefore, the difference in height between the real image and the ideal image at the intermediate portion becomes large, thereby arising a problem that the quality of a color image tends to derogate caused by color drifting in the instance of a color image or a hue (or a tint) different from an actual one.
The above-described problems should be solved by, for example, disposing a laser beam detecting sensor also at the intermediate portion between the first laser beam detecting sensor and the second laser beam detecting sensor, so as to correct to reduce the difference in height between the real image and the ideal image based on a signal detected by the laser beam detecting sensor disposed at the intermediate portion. However, then the laser beam detecting sensor disposed at the intermediate portion is located within the scanning region of the laser beam. Consequently, in this instance, the laser beam detecting sensor shuts out the scanning beam (i.e., an optical flux) directing toward a photosensitive drum, thereby making it impossible to write a desired image on the photosensitive drum.
In view of this, in the conventional image formation apparatus, the difference in height between the real image and the ideal image substantially at the center in the scanning region is detected only at the time of fabrication and correction is performed so as to reduce the difference.
However, this method has arisen a problem that it cannot cope with variations in optical characteristics caused by the deformation of an optical lens in the scanning type optical device according to variations in temperature in the environment in which the image formation apparatus is placed, or secular changes in optical characteristics caused by a long-term use.
Moreover, the image formation apparatus disclosed in Japanese Patent Application Laid-open No. H6-320786 is configured such that the writing frequency is corrected based on the difference in time when the two laser beam detecting sensors detect the laser beams, so that although the correction can be performed when a scanning speed of the laser beam is constant and is deviated from an ideal speed, the correction cannot be performed very accurately when the scanning speed is varied at each position.
It is an object of the present invention to provide a technology which makes it possible to prevent any generation of a portion at which the difference in height between the real image and the ideal image over the entire scanning region is extremely large, and further, make it difficult to generate color drifting on the image even when the color image is formed and making the hue (tint) excellent.
The scanning type optical device according to one aspect of the present invention is of a tandem type and makes it possible to print in an overlapping manner each of images formed by scanning with the plurality of scanning beams on a single piece of recording medium. This scanning type optical device comprises a deflector which deflects, with a rotating member, an optical flux emitted from a light source to obtain a plurality of optical fluxes; a plurality of scanning type optical systems each including an image focusing optical system, wherein each of the scanning type optical systems obtains a scanning beam by making each of the optical fluxes deflected by the deflector pass through the image focusing optical system; a plurality of optical characteristics measuring units each of which measures optical characteristics of the respective scanning beam.
Each of the optical characteristics measuring units includes a first optical characteristics measuring unit which detects the optical fluxes directed toward a predetermined position around one end within a scanning region of the scanning beam;
a second optical characteristics measuring unit which detects the optical fluxes directed toward a predetermined position around the other end within the scanning region of the scanning beam; and at least one third optical characteristics measuring unit interposed between the first optical characteristics measuring unit and the second optical characteristics measuring unit.
The scanning type optical device further comprises a modulating unit which modulates the optical fluxes emitted from the light sources based on signals detected by the first, second and third optical charateristics measuring units.
The image formation apparatus according to another aspect of the present invention comprises the scanning type optical device according to the present invention.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.